Steric effects of the alkyl groups: evaluation on isolated molecules by means of isodesmic reactions

Several possible scales of steric effects of the alkyl groups were suggested on the basis of isodesmic model reactions, in which a sterically crowded compound is formally synthesized from simpler derivatives. The reaction energies were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d.p)//B3LYP/6-311+G(d.p) for 6 model systems and 7 various alkyl groups. The most important systems were cis-1,2-dialkylcyclopropanes 1 synthesized from two mono derivatives and sterically crowded derivatives of bicyclo[2.2.2]octane 2 with C₃ symmetry. The scales of steric effects evaluated from the two models were rather different: the first scale depended in effect only on the C atoms in the α and β positions and the effects were almost equal for all primary alkyls. The second scale depended also on the γ position and the effect of the CH₂–t-Bu group was much greater than that of the ethyl group. Any relationship between various systems was found rarely, only in the case of very similar reaction series; even in such cases the relationship was sometimes linear, sometimes distinctly curvilinear. It is concluded that any universal scale of steric effects is in principle not possible since these effects depend specifically on the surroundings of the substituent in a particular reaction. Nevertheless, there is a similarity between various scales; a bulky group appears as bulky in any scale. Therefore, very rough correlations of steric effects are possible.